1. Field of the Invention
The present invention relates in general to an apparatus for controlling an engine of a motor vehicle and a lock-up clutch disposed between the engine and an automatic transmission. More particularly, the invention is concerned with a technique for assuring engagement of the lock-up clutch by such control apparatus during deceleration of the vehicle.
2. Discussion of the Related Art
A lock-up clutch is provided on a motor vehicle with an automatic transmission, so that an engine of the vehicle is directly connected to the automatic transmission when the lock-up clutch is in the engaged state. For such motor vehicle, there is known an engine control apparatus capable of controlling the vehicle engine in a "fuel cut" mode when the vehicle is decelerating, for example, when the vehicle is coasting with the accelerator pedal placed in its non-operated position. In the fuel cut mode, a fuel supply to the engine is cut as long as the engine speed is higher than a predetermined fuel-cut speed. On some motor vehicles, such engine control apparatus is adapted to engage the lock-up clutch so that the engine speed is held higher than the fuel-cut speed for a longer period of time, for cutting the fuel supply to the engine for an accordingly long period of time. An example of such engine control apparatus is disclosed in JP-A-60-179566 (published in 1985).
According to the known engine control apparatus indicated above, the lock-up clutch is brought to the fully engaged state when a throttle sensor detects that the throttle valve is placed in its fully-closed position. In this condition, the speed of the input shaft of the automatic transmission is higher than the engine speed, and a torque is transmitted in a reverse direction from the drive wheels to the engine when the lock-up clutch is engaged. Generally, the lock-up clutch is provided with a damper which is so designed as to permit smooth engagement of the lock-up clutch when the engine speed is higher than the input shaft speed of the automatic transmission. Accordingly, when the engine speed is lower than the input shaft speed of the transmission as in the case of vehicle deceleration or coasting with the throttle valve fully closed, the lock-up clutch cannot be smoothly engaged. Difficulty of the engaging action increases with an increase in the speed difference between the input and output coupling members of the lock-up clutch.